linux-kernel - Re: [PATCH] printk: robustify printk

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Message-ID: <20080808203049.GG6760@linux.vnet.ibm.com>
Date:	Fri, 8 Aug 2008 13:30:49 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	torvalds@...ux-foundation.org, mingo@...e.hu, tglx@...utronix.de,
	marcin.slusarz@...il.com, linux-kernel@...r.kernel.org,
	davem@...emloft.net, rostedt@...dmis.org
Subject: Re: [PATCH] printk: robustify printk

On Fri, Aug 08, 2008 at 12:14:28PM -0700, Andrew Morton wrote:
> On Fri, 08 Aug 2008 20:14:28 +0200
> Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
> >  void wake_up_klogd(void)
> >  {
> > -	if (!oops_in_progress && waitqueue_active(&log_wait))
> > -		wake_up_interruptible(&log_wait);
> > +	unsigned long flags;
> > +	struct klogd_wakeup_state *kws;
> > +
> > +	if (!waitqueue_active(&log_wait))
> > +		return;
> > +
> > +	local_irq_save(flags);
> > +	kws = &__get_cpu_var(kws);
> > +	if (!kws->pending) {
> > +		kws->pending = 1;
> > +		call_rcu(&kws->head, __wake_up_klogd);
> > +	}
> > +	local_irq_restore(flags);
> >  }
> 
> Note that kernel/rcupreempt.c's flavour of call_rcu() takes
> RCU_DATA_ME().lock, so there are still code sites from which a printk
> can deadlock.  Only now, it is config-dependent.

The RCU callbacks are (and must be) invoked without holding any RCU
locks, so printk()s in RCU callbacks are perfectly permissible.  It would
not be -that- hard to eliminate the lock in call_rcu(), but yeccch...
It would be far easier to debug RCU without the benefit printk()s than
to deal with a preemptable RCU implementation that was based solely on
atomic instructions!!!

> >From a quick look it appears that large amounts of kernel/rcupreempt.c
> are now a printk-free zone.

The only printk() is in preemptable RCU's rcu_init() function, whose
only purpose is to mark the dmesg appropriately, and which should not
be a problem.

And that would be because I do a fair amount of prototyping and debugging
in user space, where one instead uses printf().  ;-)

							Thanx, Paul
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